They weren't honest with themselves. High interest is difficult to pay. Only luck would have allowed them to pay
Ok, that is a little bit of a stretch for honesty... by that definition, every single shortcoming is actually about not being honest....
They were up to date on their payments. There was no hint they were ever going to default. It's pure supposition - of the blame the victim kind - to imply they were running some kind of inverse scam, even subconsciously, or that they would have stiffed the lender.
The article indicates they had never missed a payment.
'naivety' would probably be better. 'over optimistic'.